Lasers are ubiquitous devices used for testing, measuring, printing, cutting, marking, medical applications, communications, data transmission, semiconductor processing, and many other applications. Many types of lasers have been developed to meet different performance criteria for different applications. Engraving, cutting, marking, printing and many other applications require relatively compact lasers that generate high power output and have beams with a desired shape and energy distribution. Slab lasers are often useful in such applications because they can generate high power output in a relatively compact package.
Gas slab lasers generally have a gas containment structure, a pair of elongated electrodes juxtaposed to each other across a gap, and mirrors at each end of the electrodes forming a laser resonator. Slab lasers also have an active laser medium in the volume between the electrodes that defines the “slab.” In operation, slab lasers generate a beam of coherent light by extracting energy from an energized active laser medium using a laser resonator.
Although slab lasers are useful for many applications, it is difficult to extract a beam of good quality. More specifically, because the active laser medium has a rectilinear configuration, it produces an elliptical beam with different properties along a minor axis in the direction of the slab height and an orthogonal major axis in the direction of the slab width. In the narrow direction corresponding to the slab height, the reflecting surfaces of the electrodes can create a waveguide that defines the structure and divergence of the beam. Whereas in the orthogonal direction corresponding to the slab width, the beam is not restricted by the electrodes such that the properties of the beam are mainly defined by the properties of the laser resonator.
Extracting a good quality high-power laser beam from a slab laser is a complex problem that has been the subject of numerous inventions for many years. Several U.S. patents disclose devices and processes that attempt to obtain a good quality beam. For example, U.S. Pat. Nos. 4,719,639; 5,123,028; and 5,353,297 disclose different types of stable and/or unstable resonators for slab lasers that seek to improve the beam quality. The lasers in accordance with these patents, however, still produce elliptical beams with different divergence values along the orthogonal axes.
Other types of laser resonators have been developed to produce a high quality coherent beam from a slab that has a non-circular shape. For example, resonators disclosed in U.S. Pat. Nos. 4,972,427 and 5,608,745 use the Talbot effect for efficient selection of a single mode. Although the lasers disclosed in these patents produce high power outputs, their beam characteristics may not be acceptable for many material processing applications.
Another aspect of slab lasers is generating a high power output in a compact laser. U.S. Pat. No. 5,661,746 issued to Sukhman et al. discloses a multiple pass stable resonator that generates a high power output with good beam quality from a slab laser. The slab laser disclosed in U.S. Pat. No. 5,661,746 is a free-space laser that eliminates, or at least substantially mitigates, the waveguide effect of the electrodes. Additionally, the devices and methods disclosed in U.S. Pat. No. 5,661,746 produce a high power output because the beam propagates along multiple passes between the optical elements to effectively use the active laser medium. However, due to development of an internal parasitic mode as the number of passes inside the lasers is increased, lasers of this type are limited in power output.